In the conventional optoelectronic component, for example in conventional phosphor conversion warm white LEDs, the used phosphors are generally mixtures of yellow emitting and red emitting phosphors. The phosphor mixtures are mixed with a polymer material, such as silicon or epoxy. The phosphor mixtures are cast, deposited or coated on a blue excitation source chip in LED packages. The phosphor mixture adsorbs at least a part of the blue light from the chip and converts it to yellow and red light, creating a warm white light. The phosphor polymer mixture approach is easy to apply to LED packages and relatively less expensive. However the polymer materials are instable and degrade under high temperature and high light intensity.
There are also attempts to use glass to replace polymer matrix materials to overcome stability problems, such as examples of phosphor-in-glasses (PiGs). Most phosphor-in-glasses approaches are made when the glass softens or melts, to assure homogeneity of phosphor dispersion in glass and get rid of the majority of pores in it. On the current market there are cool white PiG LEDs available which use single yellow YAG:Ce phosphor. YAG:Ce is an oxide phosphor. It is relatively resistant to damage from glasses which are normally a mixture of different oxides. However, there are no warm white PiG LEDs on the market. This is related to phosphors used in the warm white LEDs. In addition to yellow YAG:Ce phosphor, a red phosphor is also needed. The most common red phosphors are nitride phosphors, such as (Ca, Sr, Ba)2Si5N8:Eu and CaAlSiN3:Eu phosphors. These nitride phosphors are very easily damaged by oxide from the glasses because the nitride phosphors are very sensitive to oxygen impurity. SiO2-Na2O-Al2O3-CaO glass was used to contain red nitride phosphors (“Novel broadband glass phosphors for high CRI WLEDs” Optics Express, Vol. 22, Issue S3, pp. A671-A678 (2014)). Since the SiO2-Na2O-Al2O3-CaO glass has to melt or soften at above 680° C., the red nitride PiGs made from this glass have a very low efficiency, which is indicated by means of damage to the glass. The degree of damage increases as the processing temperature increases.
It is important to reduce the processing temperature to avoid or mitigate damage to red nitride phosphors. Thus choosing glasses that melt or soften at low temperature is important. It is normally desirable to keep the processing temperature below 400° C. when processing nitride phosphor-in-glass. However, the low melting temperature glasses are normally not water-resistant which is a must-have requirement for conversion materials if they are used in LEDs or optoelectronic components. The difficulty in finding a suitable low-melting glass with good water resistance is a major technical challenge in making warm white conversion elements.